The field of the invention is washers, and the invention relates more specifically to washers of the type typically used in the construction or assembly of metal structures. Such washers are usually held to a hex head screw and are designed for use where the screw head and washer are exposed to weathering
Commonly used washers for this use typically have either a flat or domed washer with a separate or bonded neoprene or other elastomeric washer positioned on the side of the washer away from the screw head. When the screw and washer assembly is tightened, the elastomeric washer expands and helps prevent water leakage around the washer. Although a flat washer could be used with a bonded or unbonded elastomeric washer, it has been found preferable to use a domed or dished washer so that the elastomeric washer tends to be forced inwardly against the shank of the screw. This helps eliminate leakage between the screw and the elastomeric washer.
With increased labor costs, there is a continual search for ways to speed up the construction of buildings and other objects. One way of increasing efficiency has been to asssemble the screw and washer so that the user does not have to join these two elements during use. Also, since the elastomeric portion of the washer should always be adjacent the metal surface through which the screw is driven, preassembly eliminates the possibility of reversal of the washer. The assembled screw and washer is typically driven with an electric or pneumatic screwdriver of the type having a preset or adjustable amount of torque using an overruning clutch. In use, the screw head is inserted into the driving portion of the screwdriver and the point of the screw is inserted through a predrilled hole or in the event the screw is of the type having an integral drill, the point of the screw is placed in the desired location and the screwdriver is started. The screw will be driven until the torque reaches the preset level for the screwdriver. At this point, the clutch of the screwdriver will slip and the operation is complete.
Frequently, the amount of torque generated by the screwdriver exceeds the desired amount. In the past with a domed washer, occasionally the amount of torque exerted on the washer would indent the center section and cause the neoprene inner washer to spin out or otherwise tear away from the screw. This resulted in a leak and the liklihood of corrosion. A washer is therefore needed which can accept higher torque without causing the washer to tear away from the screw. Furthermore, undertorquing has led to an improper seal with presently used washers and there is a need for a washer capable of forming a water-tight seal with less than normal torque. Undertorquing is a particularly common problem when two sheets are being stitched.